Union of igraph objects loses attributes

Question

I have two igraph objects, which have different color attributes. Vertices "A" and "B" in first graph are colored red. Vertices "AA" and "BB" in second graph are colored green. After joining the two, the different colors are lost.

library(igraph)

graph.1= graph.data.frame(data.frame(start=c("a", "b"), end=c("A", "B")))
V(graph.1)[name%in% c("A", "B")]$color= "red"

graph.2= graph.data.frame(data.frame(start=c("a", "b"), end=c("AA", "BB")))
V(graph.2)[name%in% c("AA", "BB")]$color= "green"

graph= graph.union.by.name(graph.1, graph.2)

plot(graph)

How can I preserve the distinct colors when joining ?

Answer 1

igraph doesn't loose the colors, it stores them in $color_1 and $color_2. I think this is because in the general case there might be common vertices with different colors. What would you do then?

Try this:

V(graph)$color <- ifelse(is.na(V(graph)$color_1),
                         V(graph)$color_2,V(graph)$color_1)
plot(graph)

BTW: your code didn't run for me. I had to use:

graph <- graph.union(graph.1, graph.2, byname=T)